By Howard Schneider WASHINGTON (Reuters) – Federal Reserve Chair Jerome Powell used his first four years as the world’s top central banker to reshape U.S. monetary policy around the idea that low inflation and low unemployment could coexist. It was a move intended to spread the gains of economic growth more widely and keep a focus on jobs during the rebound from the pandemic. But the assumptions on which it rested – a relatively frictionless global economy with a well-greased supply chain; a balanced U.S. labor market with just over one open job for each unemployed person – have been shattered…